This invention relates to a process for the catalytic isomerization of light hydrocarbons. More particularly, this invention relates to a process for the catalytic isomerization of light hydrocarbons wherein the hydrocarbon stream contacts a catalyst comprising beta zeolite and a noble metal whereby the stream is upgraded to a product having an increased octane number.
Natural straight run gasoline (i.e., naphtha, or light naphtha) contains chiefly normal paraffins such as normal pentane and normal hexane, which have relatively low octane numbers. Representative clear octane numbers are set forth below in Table I.
TABLE I ______________________________________ Octane Number Research Motor Paraffin Clear Clear ______________________________________ n-Pentane 61.7 61.9 2-Methylbutane 92.3 90.3 2,2-Dimethylbutane 91.8 93.4 2,3-Dimethylbutane 103.5 94.3 2-Methylpentane 73.4 73.5 3-Methylpentane 74.5 74.3 n-Hexane 24.8 26.0 ______________________________________
It is of great importance to convert these low octane components to their higher octane isomers in order to supply the present and future requirements for production of gasoline. Gasoline octane specifications will become more difficult to meet with the complete phaseout of lead from gasoline and future projected limitations on high octane aromatic content in gasoline. Accordingly, a considerable number of isomerization processes have been proposed for isomerization of hydrocarbons in the petroleum industry, however, there is still need for more effective isomerization processes.
The isomerization of n-paraffins is generally believed to be a first-order reversible reaction that proceeds through a dual functional mechanism over molecular sieve based catalysts. Although not to be construed as a limitation on the present invention, this mechanism can be summarized as follows:
1. n-alkane.rarw.,.fwdarw.n-alkene+H.sub.2 PA0 2. n-alkene+H.sup.+ .rarw.,.fwdarw.n-R.sup.+ PA0 3. n-R.sup.+ .rarw.,.fwdarw.i-R.sup.+ PA0 4. i-R.sup.+ .rarw.,.fwdarw.i-alkene+H.sup.+ PA0 5. i-alkene+H.sub.2 .rarw.,.fwdarw.i-alkane
where H.sup.+ is the acid site on the zeolite surface and R.sup.+ is the adsorbed carbenium ion on the acid site.
Steps 1 and 5 are dehydrogenation reactions that take place on metal sites. Steps 2 and 4 involve chemisorption on acid sites to form carbenium ions. As long as sufficient metal sites are present and chemisorption on acid sites is facile, Step 3 is rate controlling, and Steps 1-2 and 4-5 are in near-equilibrium.
It is well recognized that the isomerization reaction is constrained by thermodynamic equilibrium between the normal paraffinic feedstock and the various isomers of this feedstock. Refiners target an economically desirable isomerate product by evaluating such parameters as the value of gasoline octane, yield, and facility cost, and develop or select processes that most economically meet their requirements. In order to ensure that refiners can maximize their degree of approach to the ideal equilibrium state of the reaction, refiners must consider the particular catalyst employed, the required feedstock preparation, and key process variables such as reaction temperature and pressure.
The isomerization reaction is particularly dependent on reaction temperature as is apparent in the case of light hydrocarbons such as hexane. If the isomerization reaction is carried out at high temperatures (over 600.degree. F.), the doubly branched isomers are much less favored than singly branched isomers or n-hexane whereas at lower temperatures (below 400.degree. F.), there is a rapid increase in the equilibrium concentration of the high octane isomer 2,2-dimethylbutane (J. A. Ridgway, Jr. and W. Schoen, ACS Symposium, Div. of Petroleum Chemistry, Boston, Apr. 5-10, 1959, A-5-A-11). FIG. 1 shows the composition-temperature equilibrium curves of the vapor phase hexane isomers as determined by Ridgway and Schoen. The curves show the more favorable equilibrium at lower reaction temperatures. However, like most catalytic reactions, the rate of reaction decreases as the reaction temperature is decreased, and the reaction will not closely approach equilibrium at low operating temperatures. At high reaction temperatures, hydrocracking increases rapidly, increasing the yield of undesirable lighter hydrocarbons at the expense of liquid products. High reaction temperatures can also increase the rate of carbon laydown (coking) on the catalyst resulting in catalyst deactivation. Thus, there is an optimum temperature range at which the isomerization reaction just begins to approach equilibrium while still minimizing the undesirable side reactions. For most molecular sieve-based isomerization catalysts, the optimum temperature lies in the range from 300.degree.-700.degree. F., with a preferred range from 450.degree.-500.degree. F. This optimum temperature, in general, is different for different catalysts with different activities and selectivities. A catalyst which operates at a lower optimum temperature is also able to take advantage of the more favorable equilibrium and lower rate of hydrocracking and coking at lower temperatures.
Feedstock composition is a key determinant as to catalyst selection and development and facility design. Catalyst contact with known isomerization poisons such as sulfur, nitrogen, and water can result in irreversible catalyst deactivation and poor product yields. Catalyst contact with heavy components with 7 or more carbon atoms or cyclics of 6 carbon atoms or more can result in adverse competitive reactions that either result in a lower octane product or in interference with the isomerization reactions by adsorption on the catalyst. For example, the presence of benzene in the feedstock results in strong adsorption on the catalyst acid sites which interferes with normal pentane and normal hexane isomerization. The benzene itself is saturated to form cyclohexane which has a substantially lower octane. The presence of heavy components with 7 or more carbon atoms also strongly inhibits the normal pentane and normal hexane isomerization reaction and increases reaction space velocity in the process of being substantially hydrocracked to lower value propane and butane products. While essentially inert to the isomerization reaction, the processing of lighter hydrocarbons of 4 carbon atoms or less in an isomerization unit adversely affects isomerization yields of normal pentane and normal hexane by increasing reactor space velocity (WHSV).
Lower reaction pressures in an isomerization process are beneficial and result in a higher rate of reaction and a closer approach to thermodynamic isomerization equilibrium. Lower pressure operations also result in construction savings (i.e., lower equipment design pressures, etc.) and reduced operating costs such as compressor horsepower. However, extremely low reaction pressures can increase catalyst coking and result in catalyst deactivation. Effective isomerization processes and catalysts must address and optimize these parameters.
Most isomerization processes are categorized as either low temperature isomerization or high temperature isomerization. Low temperature isomerization processes typically feature a highly chlorided platinum on alumina catalyst which provides high catalyst activity and permits operation at lower temperatures. Adversely, these chlorided catalyst processes require higher investment costs, are more difficult to operate, and are substantially more vulnerable to contaminants and catalyst deactivation.
High temperature isomerization process catalysts typically contain platinum or other noble metal on a molecular sieve base which does not provide the level of catalyst activity of the low temperature catalyst. However, the high temperature catalyst is much more resistant to contaminants such as sulfur and water and the processing facilities are less expensive to build. Examples of catalysts used in this type of process are disclosed in U.S. Pat. No. 3,236,903 where the catalyst is a zeolite molecular sieve containing a catalytically active metal such as rhodium, U.S. Pat. Nos. 3,236,761 and 3,236,762 where the catalyst is a Y-type crystalline zeolite containing an elemental metal of Group VIII of the Periodic Table, U.S. Pat. Nos. 3,527,835 and 3,299,153 where the catalyst is a synthetic mordenite containing highly-dispersed platinum or palladium contacted with hydrocarbon in the presence of hydrogen, and U.S. Pat. No. 3,354,077 where the catalyst is a stabilized Y-sieve hydrogen zeolite composition. A particularly common catalyst in commercial practice today is the synthetic mordenite catalyst containing a highly-dispersed platinum or palladium.
Use of zeolite beta as a catalyst component has been the subject of several catalyst synthesis patents. U.S. Pat. Nos. 4,642,226, 4,554,145, 4,683,214, and 4,615,997 are all directed towards methods for preparing catalysts which include some percentage of zeolite beta.
U.S. Pat. No. 4,518,485 discloses the use of zeolite beta as a catalyst component in a lubricating oil dewaxing process. Lube oil stock is hydrotreated over a zeolite beta catalyst for the purpose of isomerizing heavy straight-chained paraffins into branched, lower pour-point isomers and hydrocracking other heavy, high viscosity lube oil components into lower viscosity components.
U.S. Pat. Nos. 4,753,720 and 4,784,745 disclose the use of zeolite beta as a catalyst component for octane improvement of refinery produced olefinic gasoline. The process treats the cracked olefinic naphtha streams produced at a Fluid Catalytic Cracking Unit or a Coking Unit at low reaction pressures and in the absence of hydrogen so as to improve the octane of the streams.
U.S. Pat. No. 4,647,368 discloses use of zeolite beta as a catalyst component for processing full range C.sub.4 to C.sub.10 naphtha. The patent discloses that the butane component is isomerized, the pentane and hexane component is isomerized, while the heptane and heavier material component is partially hydrocracked. While the process can eliminate prefractionation steps in converting the wide range of feed components into intermediate or finished products, the process generally results in lower total gasoline octane production from less effective isomerization of the pentane and hexane component, lower gasoline yield from hydrocracking gasoline boiling range material into butane and lower boiling point components, lower refinery hydrogen production from a reduction in reformer feedstock volume to light ends, and require more costly capital due to the necessity of a very large, full-range naphtha processing facility.
It is an object of the present invention to provide a light straight-run isomerization process that involves the use of a catalyst that resists catalyst deactivation due to feedstock contaminants and poisons.
It is another object of the present invention to provide an isomerization process that permits operation at relatively lower temperatures.
It is yet another object of the present invention to provide a process that increases octane upgrade without a decrease in gasoline yield, decrease in refinery hydrogen production, or the installation of higher cost processing facilities.